Vermes
Advanced Member level 4
Main features of this amplifier are good parameters and stylish design. It is equipped with input selector based on relays, what shortens the path of signal.
The main advantage of the amplifier is that it was realized in module technic. In case of a breakdown, it can be disassembled to modules during almost 20 minutes and then assemble again.
Modules:
- main power supply
- preamplifier power supply
- protector
- power amplifier
- preamplifier
- I/O selector
- VU-meter
Power amplifier:
107-k was used in this device. Voltage gain of the amplifier: about 30dB. The only change on the PCB is replacing transistors T3, T4 BF470 with transistors BF472. After assembly of the module, the quiescent current was set to 45-50mA at symmetrical supply +/-55V. PCB of the amplifier was mounted vertically to four heat sinks with dimensions of 16,5x3,5x10cm.
Power supply:
It is a classic toroidal transformer 500W (VA) with double secondary winding 2x40V and current of 7,5A (per one half of the winding). Voltage was rectified by a 35A/1000V rectifier bridge and then filtered by 6x10000uF/63V capacitors. Additional 100n/100V capacitors were soldered to the bridges. There is also a softstart system – contacts of the relay are shorted after approximately 2 seconds from turning on the device. In that time current starts to flow through limiting resistors. Delay time of the system depends on the capacity of C2 and C3 capacitors. Varistor V1 reduces the surges caused by switching processes.
Protector:
The protector is equipped with some useful features: protection against DC voltage at the output of amplifier, reduces crashes while turning on and off and additionally protects the whole against overheat by disconnecting the speakers when the temperature on the heat sink exceeds 85 degrees Celsius. The system is powered from the main transformer.
Preamplifier:
The system uses a single ECC85 tube as preamplifier. Supply cables were put in a twisted pair. System of ground in the amplifier is serial and connects with the housing near the input where there are the lowest potentials. Preamplifier operates after about 2 minutes from turning on WSLM to provide as long life of tubes as possible. Firstly glow voltage increases thanks to a pair of 22ohm 5W resistors, and then time system turns on the alternating voltage, which in turn activates the anode voltage multiplier.
Input selector:
The system was implemented on WSLM 0.1 with four linear inputs and one cassette player output. Signal from any input is stopped on relays until the selector switch does not give the signal to CD4017, which in turn controls the transistors and voltage for coil of the relay. Then the signal passes to the preamplifier and power amplifier. The system has a separate supply as the preamplifier.
VU meter:
It is a simple and reliable system to control large meters and provide interesting look.
Link to original thread - Wzmacniacz WSLM 0.1 wykonany przez 5W